[R-390] The Penultimate R-390* Ballast Replacement

[Bob Camp]

Hi

I totally agree that if you are going to do something like this an 
external box that plugs in with no mods to the radio is the way to go. 
Chopping up the IF deck or the wiring harness simply is not worth it in 
this case.

The good old style 78xxx regulators are pretty well behaved. Some of 
the newer parts are not so forgiving. The older parts generally have 
NPN transistors in an emitter follower configuration. They are stable 
into almost anything you can tie to them. The newer parts with the 
"ultra low drop out" features have PNP devices (or FET's) in a 
collector output configuration. This makes gives them a lot less 
stability than the good old parts. Both oscillation and broad band 
noise are common issues with the newer parts.

If you do go with a solid state filament supply be sure to consider the 
inrush current. A quick check with an ohm meter on a cold tube should 
give you a pretty good idea what to expect from that particular tube. 
Common wisdom (often wrong ...) is to provide 4 to 5X the running 
current for inrush. Your 300 ma supply would have to source 1.5 amps 
while the tubes warm up.

Current limit is one way to get around this. The two common options are 
constant current limiting and fold back limiting. A fold back limiter 
is not going to do any good in this situation. A constant current 
limiter actually increases the power dissipated in the regulator as it 
cuts back. Unless there is a big heat sink this generally either melts 
the device or puts it into thermal overload. If it goes into thermal 
overload you get the same problem as with the fold back limiter.

Twelve volt one or two amp supplies are not hard to find. They also 
won't set the bank roll back by much more than a nice dinner for the 
family.

This would all be a bit easier to evaluate if we had some real data 
(1.2 Hz per 1% change) from several radios on the impact of heater 
voltage on the stability of the radio. The boys at Collins didn't take 
any data that they found convincing when they designed the radio ....

	Take Care!

		Bob Camp
		KB8TQ



On Feb 13, 2005, at 1:41 PM, John KA1XC wrote:

> Sounds good Todd.
> The 12.6 VDC supply doesn't even need to be husky, for only a 300 ma 
> load
> any of the common 78xx style 3 or 4 terminal regulators in the TO-220
> packages would suffice, and even the best regulator would only need a 
> 723
> and a pass transistor. I wouldn't even bother with connecting the 
> ground
> return to the ballast tube socket, just use any chassis ground 
> connection on
> the back panel.
>
> I've read all kinds of over-worrying about electronic regulator 
> "noise" or
> oscillations from voltage regulator IC's in radios, but it's really a
> non-issue. As long as you follow the bypassing suggestions in the app 
> notes
> for the part, use good parts, and check things with a scope it'll be 
> fine.
>
> Only *once* did I have a problem with a 3 term regulator inside an HF
> receiver. My TMC GPR-90 has a whole bunch of solid state mods and has 
> a +/-
> 12 VDC regulated supply under the chassis to run them. After I 
> installed it
> I heard some 300 kHz carriers that I didn't hear before. It turned out 
> to be
> one of the 3 terminal regulators oscillating (and I hadn't followed my 
> own
> advice about checking it with a scope!) which happened to be a TI 
> part. Took
> it out and installed a Motorola device in its place and it's been 
> clean for
> years.
>
> John
>
>
>
>
> ----- Original Message -----
> From: <ToddRoberts2001 at aol.com>
> To: <r-390 at mailman.qth.net>
> Sent: Sunday, February 13, 2005 12:54 PM
> Subject: Re: [R-390] The Penultimate R-390* Ballast Replacement
>
>
>> Most of the 3TF7 substitutes I have read about involve buildng 
>> something
>> small enough to plug directly into the 3TF7 socket or on a nearby 
>> bracket.
> This
>> comes with problems of heat dissipation or else installing unsightly
> brackets
>> near the I.F. subchassis. Some of the recent ideas got me thinking - 
>> Why
> not
>> build a small separate power supply module with a
>> well-regulated/filtered/bypassed 12.6 VDC output that could be placed 
>> next
> to the receiver and the only
>> connection would be a small umbilical cable with a 9-pin plug that 
>> plugs
> directly
>> into the 3TF7 socket? You could run the umbilical through the side of 
>> the
> R390A
>> chassis thru one of the large holes and tuck the power supply and 
>> cord out
> of
>> the way next to the receiver. This layout is similar to the way some
>> audiophile preamps use a separate power-supply module with an 
>> umbilical.
> You could
>> modify one or two pins of the 3TF7 socket with a jumper to ground to
> provide a
>> ground return for the 12.6VDC so you wouldn't have to tie down a 
>> separate
> ground
>> lead with a terminal and screw somewhere else on the chassis. When you
> plug
>> in the umbilical it breaks the 25.2 VAC circuit and connects the 
>> 12.6VDC
>> circuit and ground. The ground pins would have no effect on the 
>> original
> 3TF7 if you
>> wanted to plug one back in.  This way you could build a nice little 
>> husky
>> separate  regulated/filtered/bypassed 12.6 VDC power supply and not 
>> have
> to
>> miniaturize it or compromize the performance. If you want to go back 
>> to a
> 3TF7 just
>> unplug the DC supply 9 pin plug and put the 3TF7 back in. No unsightly
>> permanent wires or brackets hanging off the I.F. subchassis.  The
> regulated DC
>> supply should give the ultimate in stability and pure DC on the 
>> filaments
> of the
>> BFO/PTO tubes. 73 Todd WD4NGG
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